1. Field of the Invention
The Invention is a control system for a compound aircraft. For trimmed flight, the control system selects a combination of trim control settings for the redundant controls of the compound aircraft to achieve a pilot's command consistent with a user-selectable objective, such as speed maximization, fuel consumption minimization, vibration reduction or lifecycle cost reduction. For maneuvering flight, the control system distributes control among the redundant control options of the compound aircraft and may perform that distribution consistent with the user-selectable objective. The Invention is also a method for controlling a compound aircraft.
2. Description of the Related Art
A ‘compound’ aircraft is an aircraft that includes features of both fixed wing aircraft and rotary wing aircraft. The compound aircraft includes the elements of a helicopter, including at least one main rotor and a mechanism to overcome the torque response of the rotating main rotor. The compound aircraft also includes elements of a fixed-wing aircraft, such as a wing. The wing may be equipped with ailerons, flaps or a combination of flaps and ailerons known as ‘flaperons.’ The compound aircraft may be equipped with a separate thrust mechanism to drive the aircraft forward, such as a propeller in a ducted fan. Through the use of appropriate vanes or sectors that change the configuration of the duct, the ducted fan may serve as the mechanism to overcome the torque response of the rotating rotor blades and to provide yaw control.
A compound aircraft offers several advantages over a conventional helicopter. Those advantages include achieving higher flight speeds and delayed onset of retreating blade stall and leading blade compression effects. Although the advantages of a compound helicopter are well known, no compound helicopters have been placed in regular operation in commercial or military fleets. One reason is the control complexity of the compound aircraft.
The pilot of a conventional helicopter has only limited controls. The controls available for a conventional helicopter having a single main rotor and a tail rotor are:
Throttle—The pilot can control the amount of power supplied to the rotor blades and to the tail rotor.
Collective pitch—The pilot contemporaneously can change the pitch of all main rotor blades by an equal amount using the collective pitch control, also known as the ‘collective.’ Contemporaneously changing the pitch angle of all main rotor blades increases or decreases the lift supporting the helicopter. Increasing the collective and the power will cause the helicopter to rise. Decreasing the collective and the power will call the helicopter to sink.
Cyclic pitch—The pilot may use the cyclic pitch control, also known as the ‘cyclic,’ to cause the pitch angle of the main rotor blades to change differentially as the main rotor rotates through 360 degrees. The cyclic pitch control is used to control the pitch and roll of the helicopter. For example, increasing the pitch angle of a rotor blade when the rotor blade is retreating toward the rear of the helicopter and decreasing the pitch angle when the rotor blade is advancing toward the front of the helicopter will cause the main rotor plane of rotation to tilt forward and hence will cause the helicopter to move forward.
Tail rotor pitch control—For a conventional helicopter having a tail rotor mounted on a boom, a pedal-operated yaw control changes the pitch of the tail rotor blades so that the tail rotor presents more or less force countering the torque response of the rotating main rotor. The pitch of the tail rotor blades therefore controls the yaw of the conventional helicopter.
For a conventional helicopter and for a particular throttle setting, there is only one combination of trim control settings for the collective, cyclic and tail rotor pitch controls to achieve any particular desired trimmed condition of the helicopter. The pilot of the conventional helicopter therefore has few control options.
A compound aircraft will have the aforementioned controls and in addition will have other controls. For example, the compound aircraft may feature the following controls:
Flaperon controls—The flaperons (a combination of flaps and ailerons) are located on the wings. When deflected differentially like ailerons, the flaperons may cause the aircraft to roll. When deflected in unison like flaps, the flaperons may increase or decrease lift generated by the wing. In hovering flight, the flaperons may be deployed to reduce the effective wing area and hence reduce the downward force on the wings from the downwash of the main rotor.
Forward thrust control—The compound aircraft may be equipped with a ducted fan or other mechanism to provide forward thrust. Thrust provided by the ducted fan or by another mechanism that is not the main rotor is referred to in this application as “non-rotor forward thrust.”
Rudder/stabililator—The compound aircraft may be equipped with a rudder and with an elevator or stabilator. The rudder controls the yaw of the aircraft, in cooperation with the tail rotor, ducted fan, or other mechanism countering the torque reaction of the rotating main rotor. An elevator or stabilator controls the pitch of the compound aircraft, in cooperation with or instead of the cyclic pitch control.
The pilot of the compound aircraft is presented with a variety of control combinations to achieve a desired flight condition. For example, if the pilot desires to increase the forward speed of the compound aircraft, the pilot can increase the non-rotor forward thrust using the forward thrust control, can use the cyclic pitch, stabilator and throttle controls to pitch the aircraft forward, or can use any combination of forward thrust control, stabilator, cyclic pitch control and throttle. Each of the possible combinations of trim control settings offers advantages and disadvantages. A combination of trim control settings that is optimal for one objective (for example, minimizing fuel consumption) may not be optimal for another objective (for example, minimizing vibration).
Only one combination of trim control settings for the compound aircraft will be optimal for achieving a particular trimmed condition or for implementing maneuvering flight commands consonant with also achieving a particular operational objective. The prior art does not disclose a control system for a compound aircraft that allows selection among a plurality of objectives and that then automatically optimizes control settings to achieve pilot control commands consistent with the selected objective.